We describe social psychology’s major research findings on metacognition. Although metacognitive ideas have been applied to various topics in social judgment, we focus on some of the most heavily researched areas of social psychology in which the role of metacognition has been examined. These are memory and cognitive fluency, attitudes and persuasion, the self and individual differences, and bias correction. Consistent with most prior literature, we defined metacognition as second-order thoughts, or thoughts about our primary thoughts or thought processes. That indeed constitutes a basic principle in human cognition: Principle 1—There is primary and secondary cognition. Primary thoughts are those that occur at the direct level of cognition, involving initial associations. Following a primary thought, people can also generate other thoughts that occur at a second level which involve reflections on the first-level thoughts or the process that generated these thoughts. Principle 2—Second-order cognition can magnify, attenuate, or even reverse first-order cognition. Principle 3—Second-order thoughts can be coded into the same categories that have already proven effective for classifying primary thoughts, such as target, evaluation, number, and confidence. Principle 4—The content and process bases of metacognitive judgments are likely to be as consequential as are the bases of primary cognition. Principle 5—Although explicit metacognitive activity is generally more likely to take place when people have the motivation and ability to attend to and interpret their own cognitive experiences, metacognition might also operate outside awareness with important consequences for social judgment and behavior. Issues for future research are addressed.